If a majority of their data storage needs are actually handled by third party entities, are those facilities required to meet this?.
You make a good point either way, but I do wonder if it's fair to say that this represents a "majority of their data storage needs."
Consider that, with over a billion active iPhones worldwide as of earlier this year, that means at least a billion active iCloud accounts — since very few people own more than one iPhone, it's fair to say that every iPhone represents at least a free iCloud account.
Since every iCloud account has at least 5GB of free storage, that means that even if Apple didn't have a single paying iCloud user, it would still need 5 exabytes of storage to accommodate those accounts. However, analyst reports over the past five years or so have estimated that around 20% of iCloud accounts are on a paid tier. Since these start at 50GB, this means that around 200 million of these accounts make up at least an extra 9 exabytes.
So, even if nobody was paying for anything more than 50GB, that's a minimum of roughly 14 exabytes. This would make the Google portion the "majority," but of course, a good chunk of these 200 million paid accounts are almost certainly paying for more than 50GB of storage, and since the higher tiers are 200GB and 2TB, the storage requirements would just balloon from there.
It's probably almost impossible to know what the breakdown is in terms of storage tiers, but just for the sake of discussion, let's say that 5% of iCloud users pay for the 200GB plan, and only 1% of all iCloud users opt for the 2TB plan. In that case, Apple's iCloud storage requirements would work out to over 41 exabytes, at which point the 8 exabytes that it's buying from Google is really just extra capacity beyond what Apple is yet able to handle in its own data centres. This seems likely as well since Apple is still investing in data centres, but it clearly just can't build them fast enough, when you consider that this report says that its storage requirements from Google grew over 50 percent in the past year alone — perhaps bolstered by Apple One plans and the end of the Google Photos free ride.
This is also a fairly conservative estimate, since there are also iCloud users who are not iPhone users, so they'd be above and beyond those accounted for by the billion active iPhones. Further, while the 200GB and 2TB plans do allow for family sharing, they're also more likely to encompass these non-iPhone iCloud users, such as kids with iPads, iPods, and Family Setup Apple Watches.